1. Field of the Invention
The present invention generally relates to a sheet conveying device and an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus, such as a laser printer, sheets, such as transfer paper, loaded in a feeding device are fed off one by one and conveyed onto a transfer position. At the transfer position, a position of a toner image formed on, for example, a photosensitive drum or a photosensitive belt, and a transfer position on the sheet side are aligned with each other, so that the toner image is transferred onto the sheet. After the transfer, the sheet is output as a reproduction having the toner image fixed therein.
With the aim of transferring the image at a correct position relative to the sheet, such an image forming apparatus includes a registration mechanism that aligns a sheet feeding timing with a timing at which the image arrives at the transfer position.
A known configuration for a registration mechanism (for example, Japanese Utility Model Application Laid-open Publication No. S64-000555) includes a gate member disposed, in a sheet conveying direction, upstream of registration rollers used for feeding out a sheet according to a registration timing. The gate member can advance into, or retract from, a sheet conveying path.
In this configuration, conveyance of the sheet is temporarily continued with a leading end of the sheet abutted against the gate member to thereby bend part of the sheet and form a curved portion. This enables the leading end of the sheet in which the curved portion is formed to advance toward a nip between the registration rollers by using an action to extend the curved portion by a shape restoring force of the sheet generated when the gate member retracts from the sheet conveying path.
Use of the shape restoring force of the sheet allows a clamping position of the leading end of the sheet by the registration rollers to be uniform at all times among different sheets. This enables an image position arrival timing and the sheet feeding timing toward the transfer position to be aligned with each other constantly among different sheets that are continuously conveyed.
The configuration that incorporates the gate member requires a gate member that is specially prepared in addition to the registration rollers. This increases the number of components of the sheet conveying device, which complicates a configuration of the sheet conveying device and invites increased cost including assembly.
Another arrangement has been developed in which a registration roller includes a leading end abutment mechanism (for example, Japanese Patent Application Laid-open No. H5-338865). Japanese Patent Application Laid-open No. H5-338865 discloses a configuration that includes a torque limiter interposed in a rotating shaft of the registration roller and a gate member disposed rotatably on the rotating shaft.
In this configuration, the gate member is operatively associated with forward and backward rotation of the registration roller and oscillatable between a position at which a sheet leading end is to be positioned, specifically, a position against which the sheet leading end is abutted, and a position at which the sheet having the leading end abutted against the gate member can be fed off, specifically, a retracted position.
The gate member is operatively associated with rotation of the registration roller in a direction opposite to a sheet feeding direction, thereby oscillating to the position at which to abut against the sheet leading end. The gate member is operatively associated with rotation of the registration roller in a direction of the sheet feeding direction, thereby oscillating to the position retracted from the abutment position. Upon collision with a sheet conveying guide member, the gate member remains stationary at the position with no rotation being transmitted through operation of the torque limiter.
Japanese Patent Application Laid-open No. 2012-030971 discloses a sheet conveying device that includes a registration roller and a gate member disposed coaxially with each other to thereby achieve a simple configuration. The sheet conveying device is capable of aligning the toner image on a recording medium with the transfer position accurately and within a short time, the recording media being conveyed at high speed and at short intervals.
In general, in sheet conveying devices, print skew may not be properly corrected due to, for example, a pair of registration rollers or a gate member being out of a correct mounting position, a warped shaft, and part-to-part variations.
Additionally, an image forming apparatus may be installed on a distorted or warped surface. Such an image forming apparatus may be affected by the improper mounting surface, causing a reference line of a skew correcting mechanism to be distorted relative to an image transfer unit or the image transfer unit itself to be distorted, resulting in image skew.
An error in skew correction may also occur due to wear or a change with time in a supporting member of a shaft of the pair of registration rollers.
The image forming apparatus may also be operated to form an image on top of a sheet on which an image has previously been formed. In such a case, the image originally formed on the sheet may not be square relative to the sheet, specifically, the image may be recorded slantwise relative to the sheet. This requires that an arrangement be made to intentionally give the sheet a predetermined amount of inclination for its conveyance. Such an adjustment is difficult to make, which poses a problem.
Various techniques have been developed that detect and adjust such skew. Japanese Patent Application Laid-open No. 2001-335166, for example, discloses a paper feeding mechanism that includes a paper feeding roller driving shaft disposed in a frame in a manner being inclinable relative to a normal sheet conveying direction and a changeover lever that is capable of adjusting an inclination angle of the paper feeding roller driving shaft, the changeover lever being oscillatably moved to thereby allow sheet conveyance to be performed in a condition of being obliquely supported at any desired angle. In addition, Japanese Patent Application Laid-open No. 2009-057143 discloses a feeding device that similarly adjusts an angle of a roller driving shaft to thereby adjust skew in a recording medium in a direction orthogonal to a conveying direction.
In addition, Japanese Patent Application Laid-open No. 2008-239340 and Japanese Patent Application Laid-open No. 2010-024059 each disclose a conveying device that includes an abutment member (gate member) that blocks a conveying path of a recording medium to thereby allow a leading end of the recording medium to abut thereagainst and opens thereafter the conveying path and a variable unit that varies inclination in a width direction of the abutment member relative to a conveying direction of the recording medium, the abutment member being inclined relative to a registration roller.
The arrangements disclosed in Japanese Patent Application Laid-open No. 2001-335166 and Japanese Patent Application Laid-open No. 2009-57143, although being capable of adjusting the inclination angle of the paper feeding roller driving shaft, have no gate members, which makes it difficult to adjust the sheet feeding timing. The arrangements disclosed in Japanese Patent Application Laid-open No. 2008-239340 and Japanese Patent Application Laid-open No. 2010-24059 do have a gate member and its inclination angle is adjusted; however, the arrangements require a gate member in addition to the registration roller, which increases the number of components of the sheet conveying device. This poses problems of a complicated configuration of the sheet conveying device and increased cost including assembly.
In contrast, referring to Japanese Patent Application Laid-open No. 2012-30971, the sheet conveying device is simply structured for improved productivity by having a unit (shift unit) supporting the gate member and a pair of clamp carriage rollers (a pair of registration rollers). The arrangement disclosed in Japanese Patent Application Laid-open No. 2012-30971 controls movement of the shift unit in an axial direction (a direction orthogonal to the sheet conveying direction) to thereby permit lateral shift motion of the gate member. Nonetheless, the arrangement disclosed in Japanese Patent Application Laid-open No. 2012-30971 poses a problem in that the inclination angle of the gate member cannot be easily adjusted.
Therefore, there is a need to provide a sheet conveying device having an arrangement in which a gate member and a pair of registration rollers are supported by a shift unit with its movement controlled in a direction orthogonal to a sheet conveying direction, the arrangement enabling a simple adjustment of a gate angle, and an image forming apparatus having the sheet conveying device.